1. Field of the Invention
The present invention relates to a radial line slot antenna, and more particularly to a radial line slot antenna having an antenna disk which has a structure where a feeder section is disposed on the front side of the feeder disk comprising the feeder section.
2. Description of Related Art
Along with the remarkable development of radio communication technology, frequency bands allocated to various communication equipments tend to be insufficient recently. To effectively use frequencies in this situation, the development of a technology required for shifting to higher frequency bands is now an urgent issue.
For example, millimeter wave bands, which have been used almost exclusively for basic research, are now used for Intelligent Transport Systems (ITS). In the near future, in automobile based societies like Japan, the US and Europe, it is expected that millimeter wave band related communication equipment will be used just like home electronic equipment for general consumers.
In the above mentioned millimeter wave band communication field, it is inevitable that various electronic components and devices must be able to be used in millimeter wave bands. One of the most critical devices in this sense are antennas.
At the moment, research organizations and manufacturers world-wide who participate in the research and development of millimeter wave communication are competing in the development of high performance antennas for millimeter wave bands. Various types of configurations of millimeter wave band antennas have been developed so far. One that has very good characteristics among these antennas for millimeter wave bands is a radial line slot antenna (Reference 1: “Measurements of planar feed circuits for a radial line slot antenna”, A. AKIYAMA, J. HIROKAWA, M. ANDO, Proceedings of the 2000 IEICE General Conference, B-1-125, March 2000 and Reference 2: “A Feeding Circuit for Concentric Arrays of Radial Line Slot Antenna”, M. ISHII, T. KOSHIO, N. GOTO, Proceedings of the 2000 IEICE General Conference, B-1-128, March 2000).
This radial line slot antenna was developed as an antenna where the radiation characteristic has circular polarization.
This radial line slot antenna has many advantages, and is expected to play an important role as a millimeter wave band antenna for mobile communication, including radio LAN, in the near future.
The name of the radial line slot antenna is often simply referred to as “RLSA”, which stands for Radial Line Slot Antenna. Herein below, the radial line slot antenna is referred to as an “RLS antenna” for description, in order to prevent confusion with other electronic components.
FIG. 1a-FIG. 1c are diagrams depicting the configuration of the antenna disk in a conventional RLS antenna. FIG. 1a is a plan view depicting the configuration of the front side of the antenna disk, and FIG. 1b is a plan view depicting the configuration of the rear side of the antenna disk. FIG. 1c is a side view depicting the configuration of the side face of the antenna disk.
The conventional antenna disks 1 shown in FIG. 1a-FIG. 1c are circular printed boards, which is a dielectric having the thickness characteristic to be described herein below. In this circular printed board, metallic foil (copper foil) on both the surfaces, front and back thereof, are processed. On the front side shown in FIG. 1a, many slots 2 comprised of two slot elements 2a and 2b, which do not cross and are created by etching processing, are arranged. These many slots 2 are arranged at equal intervals on a plurality of cocentric circumferences from the center of the antenna disk 1 to the periphery direction.
These slots 2 are created by etching processing such that the dielectric of the printed board is exposed. The many slots 2 are created such that the slot elements 2a and 2b form a right angle so as to radiate circular polarized waves.
The length and width of the two slot elements, which do not cross each other in a respective slot 2, the number of slots 2 in each circumference in the circular arrangement of the many slots 2, and the number of slots arranged in the circular shape, are determined depending on the specifications to obtain the radiation characteristic of a desired RLS antenna. These specifications are related to the thickness of the dielectric on the printed board, the dielectric constant thereof and the thickness of the metallic foil.
On the rear side of the antenna disk 1, the feeder section 3 is created by etching processing at the center of the antenna disk 1, as shown in FIG. 1b. This feeder section 3 is comprised of a ring slot and a perturbation element, which will be described herein below. The shape and dimensions of the ring slot and the perturbation element in the feeder section 3 are determined by the size of the feeder section in the feeder disk disposed between the feeder section 3 and the feeder wave guide.
The metallic foil (e.g. copper foil) which covers both the front and rear surfaces of the antenna disk 1 is in ground potential. To maintain this potential status, the metallic foil is coated on the side face portion of the antenna disk 1. This is to prevent the electromagnetic waves, which propagate through the dielectric, from leaking and radiating from the side face of the antenna disk 1.
FIG. 2a and FIG. 2b are plan views depicting the general configuration of the feeder section disposed on the rear side of the antenna disk. FIG. 2a is a plan view depicting an enlarged configuration of the rear side of the antenna disk, and FIG. 2b is a plan view depicting an enlarged configuration of the feeder section 3.
The feeder section 3 shown in FIG. 2a is comprised of the ring slot 3a and the perturbation element 3b, as shown in FIG. 2b. The ring slot 3a is created with the central axis of the antenna disk 1 as the center, which is a ring shape having a predetermined width and diameter defined considering the impedance related to the operating frequency, and is created by etching the metallic foil.
The perturbation element 3b is created with the central axis of the antenna disk 1 as the center, just like the ring slot 3a, which is a rectangular shape having a predetermined length and width defined considering the impedance related to the operating frequency, and is created by etching the metallic foil. This perturbation element 3b is disposed with angle θ of inclination in the counterclockwise direction from the virtual principal line 1a in the Y axis direction in the plane of FIG. 2a (Reference 3: “A Rectangular-to-Radial Waveguide Transformer through a Ring Slot for Excitation of a Rotating Mode”, K. SUDO, J. HIROKAWA, M. ANDO, Proceedings of the 2000 IEICE General Conference, B-1-126, March 2000 and Reference 4: “Design of a Millimeter-wave Radial Line Slot Antenna Fed by a Rectangular Waveguide through a Ring Slot”, K. SUDO, A. AKIYAMA, J. HIROKAWA, M. ANDO, Proceedings of the 2000 Communications Society Conference of IEICE, B-1-62, September 2000).
The arrangement and size of the ring slot 3a and the perturbation element 3b and the angle θ of the perturbation element 3b are demanded to be highly accurate, since the input impedance of an entire RLS antenna is determined by the dimensions of the feeder section of the feeder disk, to be described herein below.
If the operating frequency is in the 40 GHz band, for example, the above mentioned angle θ is 30 some degrees, but for the accuracy of the 30 some degrees, a first decimal place level of accuracy is required.
FIG. 3a-FIG. 3e are diagrams depicting the general configuration of a conventional feeder disk. FIG. 3a is a plan view depicting the configuration of the front side of the feeder disk, and FIG. 3b is a diagram depicting the configuration of the A-A′ cutting plane (cross-section) of the feeder disk. FIG. 3c is a diagram depicting the configuration of the C-C′ cutting plane of the feeder disk, and FIG. 3d is a diagram depicting the configuration of the B-B′ and D-D′ cutting planes of the feeder disk. And FIG. 3e is a plan view depicting the configuration of the rear side of the feeder disk.
The feeder disk 4 shown in FIG. 3a-FIG. 3e is created with a 5 mm or thicker brass material. In the feeder disk 4, the entire shape, particularly the flat part, has the size and shape whereby the antenna disk 1, described with reference to FIG. 1a-FIG. 1c, can be disposed. The feeder disk 4 has a rectangular feeder section 5 which passes through the feeder disk 4 in the central axis direction. This feeder section 5 matches the ring slot 3a and the perturbation element 3b in the feeder section 3 of the antenna disk 1, and has a size with dimensions which matches the impedance in the operating frequency thereof. To avoid a confusion of terms, the feeder section 3 may be referred to as the first feeder section, and the feeder section 5 as the second feeder section respectively.
On the periphery of the front side edge of the feeder disk 4, side stoppers 6 are disposed such that the size between the inner wall faces becomes several tens of μm larger than the diameter of the antenna disk 1, as shown in FIG. 3a and FIG. 3d. 
When the antenna disk 1 is housed inside the side stoppers 6, the center of the antenna disk 1 and the center of the second feeder section 5 match. On the side face of the feeder disk 4, screw holes 7 for securing the entire RLS antenna to another device (not illustrated) are disposed, as shown in FIG. 3b and FIG. 3c. On the rear side, the pin holes 8 for aligning with the feeder wave guide, which is described later, and the screw holes for installation are disposed, as shown in FIG. 3e. 
Now the assembly of the RLS antenna will be described with reference to FIG. 4a, FIG. 4b and FIG. 5.
FIG. 4a and FIG. 4b are diagrams depicting the RLS antenna assembly status, which is shown as cross-sections, and FIG. 5 is an enlarged plan view depicting the positional relationship between the feeder section (first feeder section) of the antenna disk, the feeder section (second feeder section) of the feeder disk, and the opening of the feeder wave guide. FIG. 5 is a plan view depicting the configuration from the feeder wave guide side.
At first in FIG. 4a, FIG. 4b and FIG. 5, the antenna disk 1 is housed inside the side stoppers 6 of the feeder disk 4, so that the front face of the feeder disk 4 and the rear face of the antenna disk 1 match. In this case, the center of the antenna disk 1 and the center of the feeder disk 4 are matched by the side stoppers 6. Since the angle of the perturbation element 3b of the first feeder section 3, created on the rear side of the antenna disk 1, is not a predetermined angle θ, the positional angle of the antenna disk 1 is fine-adjusted, while checking the reflection loss characteristic of the antenna.
Also the angle of the perturbation element 3b is set to a predetermined angle θ by fine adjustment. Then the antenna disk 1 is electrically bonded to the feeder disk 4 by a conductive adhesive or a conductive adhesive sheet.
In this case, a liquid conductive adhesive or a conductive adhesive sheet, which are not initially adhesive and which are adhered by air drying or heating after alignment, are used.
Then as FIG. 4b shows, the pins 13 of the feeder wave guide 11 are inserted into the pin holes 8 created on the front side of the feeder disk 4, so that the directions of the second feeder section 5 of the feeder disk 4, where the antenna disk 1 is mounted, and the opening 12 of the feeder wave guide 11, match.
And the screws 15 are screwed into the screw holes 9 of the feeder disk 4 respectively via the four screw holes 14 created in the feeder wave guide 11 to secure the feeder disk 4, where the antenna disk 1 is mounted, to the feeder wave guide 11.
When the RLS antenna assembled in this way is viewed from the feeder wave guide side, the center of the ring slot 3a is positioned on the common central axis of the second feeder section 5 (broken line) and the opening 12, and the angle of the perturbation element 3b is a θ angle included from the virtual principal line 1a. FIG. 4b shows the RLS antenna radiation directivity 16 after completion.
For this RLS antenna of the prior art, the ring slot 3a and the perturbation element 3b disposed on the rear side of the antenna disk 1 must match with the second feeder section 5 of the feeder disk 4 at high accuracy to correctly match the impedance of the second feeder section 5. For this, the angle of the antenna disk 1 is fine-adjusted while measuring and confirming the reflection loss characteristic or the impedance characteristic of the antenna as described above. In this case, it takes an enormous amount of time for this fine adjustment.
For the fine adjustment of the angle of the antenna disk 1, the antenna disk 1 may be mounted on the feeder disk 4 using various instruments appropriate for the RLS antenna shape so as to improve accuracy thereof, but in this case as well, it takes an enormous amount of time for adjustment, and a high accuracy adjustment and decreasing the RLS antenna cost are difficult. In other words, decreasing price and increasing the performance of an RLS antenna cannot be easily implemented.